The invention relates to a drilling tool especially for the rotary percussion drilling of preferably rock.
Drilling tools and especially rock drills are used for making perforations in concrete or masonry and, in particular, are also designed for use in drilling hammers. To make relatively large perforations, so-called drill crowns, such as are represented by way of example in DE 3,322,887, are generally used. Such a drill crown consists of a pot-shaped housing with hard-metal cutting bits arranged on the end face. A concentrically inserted centering drill serves for centering, especially during the initial drilling of the rock material.
Furthermore, to make generally relatively small perforations, so-called perforation drills have become known, these generally consisting of a solid drill head, the drill shank being provided with an integral or separate conveying helix (DE 2,856,205 A1; DE 3,614,010 A1). The rock drill according to DE 2,856,205 having a pot-shaped cross-drilling head also has a separately insertable centering drill.
Both with drill crowns and with perforation drills having exchangeable centering drills, there is the problem of placing the centering drill securely, but also exchangeably in a bore in the drilling tool. For this purpose, this bore for receiving the centering drill is designed generally conically, in order to receive a likewise conical centering drill shank. However, exchange after a lengthy operation of the drilling tool may present problems, since the clamping effect of the conical faces is so great that the centering drill cannot be removed without further action. For this purpose, DE 3,322,887 provides, for example, a transverse bore, in which the end of the centering drill projects. The centering drill can then be pressed out by means of a separate tool. However, such a transversely extending recess at the end of the conical seat in the drill crown shank constitutes a disturbance of cross section which not only impairs the transmission of percussion movements but also adversely weakens the cross section of the tool shank.
Furthermore, DE 3,322,887 has, on the centering drill, a retaining element which is designed as a separate ring. However, this device serves merely to prevent the centering drill from falling out inadvertently. Normally, the centering drill is seated firmly in the drill crown as a result of the conical frictional connection.
A further known solution according to German Utility Model GM 1,992,344 provides for the centering drill of a drill crown to be fastened axially non-displaceably within the drill crown shank by means of a bayonet-like connection. For this purpose, the centering drill has, at the rear end of its chucking shank, a flattening which cooperates with a bolt located eccentrically in a transverse bore. An additional groove approximately perpendicular to the flattening is connected to the transverse bolt in the manner of a bayonet fastening by a rotation of the drill. The centering drill shank is thereby secured axially non-displaceably against falling out.
The general prior art also includes fixing the centering drill in a drill crown by means of a transverse screw. In this case, the centering drill is recessed in the drill longitudinal direction in the region of the penetrating screw, so that, where appropriate, an axial movement of the drill is allowed.
The subject of Utility Model DE-U1-85 21 577 also shows a drill crown, with an axially movable centering drill, laterally arranged setscrews projecting into the receiving bore for the centering drill. In this case, the centering drill is designed as a known "SDS drill" which has longitudinal grooves for separate rotational takeup as well as axial securing by means of elements which in each case are separate. Such a tool, admittedly, allows a cost-effective use of a known SDS drill. However, the mechanical outlay on the shank of the drill crown is considerable.
The disadvantage of the known solutions having an axially movable centering drill is also that, during rotary percussion drilling, the centering drill executes relative to the remaining tool longitudinal and rotational oscillating movements which, under high stress, lead to destruction of the holding means for the centering drill. In particular, for example, the thread of a transverse holding pin is destroyed relatively quickly as a result of these oscillations. Also, in the case of percussion stress with an axially displaceable centering drill, the multiplicity of individual parts used according to Utility Model GM 85 21 577 are exposed to high wear.
Furthermore, the fixed attachment of the centering drill, for example by means of a conical seat, has the disadvantage that the drilling capacity is markedly reduced, in comparison with an embodiment in which the centering drill can also be set in axial oscillating movement as a result of percussion stress.